Trying to gain weight, what pills can I take to gain weight, I eat about 2500 cal. A day n lift weights. On acne meds, doxy im 5'1 n weight 100.?

Gaineng weight. That is an unusal question, when most people are trying to lose weight. I started out at 165 lbs training for a marathon and wound up at 185 lbs. I switched to triatholons and wound up at 195 lbs. Now that i'm older and do not engage in such sports, i'm having a hard time getting back my 215 lbs to 195. Do not worry about how much weight you may gain or lose, during training.
Unfortunately. All we can do is eliminate disease. You need a good endocrine work up. If all is normal, than focus on fitness, and quality of calories. There is no med to increase weight which won't just increase fat and no one needs that. You certainly take in adequate calories.